A conventional sleeve (or L-shaped) antenna mounted in a wireless communication device is illustrated in FIG. 1. As shown, the antenna comprises a coaxial transmission line 10 including an inner conductor (or core) 14, an outer conductor (or shielded mesh or ground line) 16, and a cylinder 17 of insulated dielectric material sandwiched between the inner and outer conductors 14 and 16 so that a concentric conductor as known in the electromagnetism is formed by both the inner and outer conductors 14 and 16. Moreover, an insulated shell 19 is formed around the coaxial transmission line 10. The coaxial transmission line 10 has one end coupled to a control circuit (not shown) of the wireless communication device so that the coaxial transmission line 10 can be served as a feed line. A ground metal plate 18 is formed on the other end of the coaxial transmission line 10. The ground metal plate 18 is coupled to the outer conductor 16 so as to electrically ground the outer conductor 16 of the coaxial transmission line 10. An extension 12 is formed from the inner conductor 14 at the other end of the coaxial transmission line 10. The extension 12 is shaped like an inverted L shape and extends beyond the ground metal plate 18. A length of the inverted L-shaped extension (i.e., radiating element) 12 is closely related to a resonant frequency of the antenna which is typically operated at a single frequency.
For making the L-shaped antenna more compact, a technique of manufacturing the antenna on a printed circuit board is adopted by some manufacturers in the art as shown in FIG. 2. The L-shaped antenna comprises a dielectric substrate 27, a patch line 24 printed on the top of the dielectric substrate 27, the patch line 24 having one end formed as a signal feed point 241, a ground metal plate 28 printed on the bottom of the dielectric substrate 27 opposite to the patch line 24, and an inverted L-shaped radiating element 242 formed at the other end of the patch line 24, the inverted L-shaped radiating element 242 being extended in a direction perpendicular to the patch line 24 above and beyond the ground metal plate 28. A length of the inverted L-shaped radiating element 242 is closely related to a resonant frequency of the antenna which is typically operated at a single frequency.
Further, for the purpose of using a coplanar wave guide as a feed line, another technique of manufacturing the L-shaped antenna on a printed circuit board is adopted by some manufacturers in the art as shown in FIG. 3. The coplanar wave guide based L-shaped antenna comprises a dielectric substrate 37, a coplanar wave guide line 34 printed on the top of the dielectric substrate 37, the coplanar wave guide line 34 having one end formed as a signal feed point 341, two spaced ground metal plates 38 printed on the top of the dielectric substrate 37 (i.e., the same surface as the coplanar wave guide line 34) with the coplanar wave guide line 34 located therebetween and spaced apart, and an inverted L-shaped radiating element 342 formed at the other end of the coplanar wave guide line 34, the inverted L-shaped radiating element 342 being extended in a direction perpendicular to the coplanar wave guide line 34 and beyond the ground metal plates 38. A length of the inverted L-shaped radiating element 342 is closely related to a resonant frequency of the antenna which is typically operated at a single frequency.
There has been a significant growth in wireless local Area network (WLAN) due to an increasing demand of mobile communication products in recent years in which IEEE 802.11 WLAN protocol is the most important one among a variety of WLAN standards. The IEEE 802.11 WLAN protocol was established in 1997. The IEEE 802.11 WLAN protocol not only provides many novel functions for WLAN based communication but also proposes a solution for communicating between mobile communication products made by different manufacturers. There is no doubt that the use of the IEEE 802.11 WLAN protocol is a milestone in the development of WLAN. The IEEE 802.11 WLAN protocol was further modified for being adapted to serve as a standard of both IEEE/ANSI and ISO/IEC in August 2000. The modifications comprise IEEE 802.11a WLAN protocol and IEEE 802.11b WLAN protocol. In an expanded standard physical layer, the operating frequencies have to be set at 5 GHz and 2.4 GHz. As such, the well-known L-shaped antenna cannot satisfy the requirement of enabling a mobile communication product to use both IEEE 802.11a and IEEE 802.11b WLAN protocols at the same time. Instead, several antennas have to be mounted in the product for complying with the requirement of frequency band. However, such can increase a manufacturing cost, complicate an installation procedure, and consume precious space for mounting the antennas. As a result, the size of the product cannot be reduced, thereby contradicting the compactness trend.